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Background: Smoking cessation/quitting smoking is among the most important health-promoting life changes a person can make, and is the most cost-effective disease prevention intervention available. Despite the benefits, many smokers have difficulty quitting using current smoking cessation options and the incidence of smoking relapse is high. Alternative treatments that are safe, effective, relatively inexpensive, and readily available are needed. Broad-spectrum micronutrient (minerals and vitamins) supplements are one such possible alternative. Research on the use of micronutrients in the treatment of substance dependence has existed for half a century. Investigations in to the use of micronutrients to relieve psychological symptoms has grown substantially in the past decade and has now been demonstrated in a number of trials. This thesis presents two randomised control trials to investigate whether a broad spectrum mineral vitamin formula supports smoking cessation. Aim Study One: Study one, a pilot trial, investigated if a broad-spectrum micronutrient formula supports withdrawal, smoking cessation, cigarette/day reduction and associated psychological symptoms. Methods: This pilot study was a single-case multiple-baseline design with replication across participants (n=24), nested within an active treatment-placebo double-blind randomized trial. Following a baseline phase of one to three weeks when smoking history, daily cigarette consumption (cigarettes/day), withdrawal, and nicotine dependence were measured, 24 participants were randomized (12/group) to placebo or micronutrient conditions. A four-week pre-quit phase permitted titration up to 12 capsules/day and ensured the metabolic effects of the capsules were established before quitting. In the smoking cessation phase (12 weeks duration), participants registered with Quitline NZ (not including Nicotine Replacement Therapy (NRT)), read the Quit book, and continued to consume the 12/day micronutrient or placebo capsules while attempting to quit smoking from a target quit day. Participants were counted as having a quit success if they were abstinent for ≥three days after their quit day. Quit success was established through self-report at study waypoints (4, 8, and 12 weeks post quit day). Abstinence was counted if a participant had not relapsed (≥three continuous days smoking) from their quit date to the end of the study waypoint. Daily withdrawal measures were taken at weeks 1-4, 8, and 12 post quit. Monthly withdrawal and psychological measures were completed at each study waypoint. Results: The results showed that median withdrawal scores were higher in the placebo group. Drop-out before making a quit attempt was lower in the micronutrient group (0 vs 4) and those taking micronutrients were more likely to make a quit attempt (11 (91.7%) vs 5 (41.7%); Odds Ratio (OR) = 15.4; 95% Confidence Interval (CI) [1.47, 160.98]) and be successful at quitting (≥ 3 days smoke-free) (10 (83.0)% vs 4 (33.3%); OR = 10.0; 95% CI [1.44,69.26]). The micronutrient group had a higher quit rate at four weeks (58.3%) compared to placebo (25.0%), this was not significant (χ² (1, n= 24) = 2.74, P=0.10), and NNT=13. Aim Study Two: Study two, a fully-blinded randomised placebo-controlled trial was designed to further investigate the use of a broad-spectrum micronutrient formula to assist with quitting smoking, reduction of cigarettes/day, withdrawal symptoms, and associated psychological symptoms. Methods: Following the baseline phase (one to two weeks) when smoking history, daily cigarette consumption (cigarettes/day), withdrawal, and nicotine dependence were measured participants (n=107) were randomised to placebo (n=50) or micronutrient conditions (n=57). A four-week pre-quit phase permitted titration up to 12 capsules/day. During the quit phase participants were registered with a public Quitline (not including NRT), read the Quit book, and attempted to quit smoking on a target quit day while continuing to consume micronutrients or placebo for 12 weeks. Carbon monoxide levels and self-report confirmed continuous abstinence at 4, 8, and 12 weeks post-quit. Results: Twenty-two (39%) participants in the micronutrient group and twenty-one (42%) in the placebo group completed the full trial. Micronutrient + Quitline and Placebo + Quitline groups did not differ on the primary outcome of continuous abstinence at 12-weeks post-quit day using intent-to-treat analysis; however, 28% of the micronutrient treated group had quit versus 18% for placebo (OR=1.78, 95%CI [0.71, 4.48]), with NNT=10. Comparison of cigarettes/day between micronutrient and placebo groups showed that those taking micronutrients reported reduced consumption throughout the trial, notably at pre-quit weeks one and four, and at quit phase week four. There were no serious adverse events, masking was successful and there were no substantive group differences in side effects or drop-out rate. Conclusion: The two studies presented are the first RCT’s investigating the impact of micronutrients on smoking reduction, finding that micronutrients support quit attempts, and reduced harm through reduction in number of cigarettes smoked relative to placebo. The small sample sizes and high drop-out rate limits confidence in the conclusions and generalizability of the results; however, micronutrients were shown to be comparable to other smoking cessation treatments but with fewer side effects. Future research using larger and longer trials including cost effectiveness and biomarker measures is encouraged.